This study is designed to investigate the behaviors unique to early infantile autism within four are groups (birth through six months, seven through 30 months; 31 months through five years; six years through 12 years), delineate key areas of dysfunction which are predominant in autism and develop an instrument, The "Autistic Descriptors Checklist (ADC)" to be used for differential measurement purposes. The ADC will be studied and revised as necessary to achieve a level of validity, reliability, and useability for appropriate and reliable use in interviews with caretakers, observations and record reviews of clients. It will serve the function of ascertaining baseline and changing behaviors of autistic children and differentiating this population from others who may have some of the same characteristics (i.e., mentally retarded). This instrument would also be useful to delineate clients' therapeutic, educational and habilitation needs as well as indicate further areas for individual and group study. It is postulated that standardized descriptions of behaviors provided by such an instrument will allow for future studies related to characteristecs of older autistic persons (14 years through 18 years, 19 years and older), formative and summative studies of individuals including changes over time resulting from various medical or educational interventions, establishment of subgroups of autisitc children on the basis of similarity of behavioral profiles and correlations of autistic features with associated biological deviations. The need for an instrument with the above characteristics was based on a study of existing assessment materials, none of which served to accurately describe a population across all age groups or using all available data. This has resulted in inability to interprete research findings because there is no one set of behaviorally described diagnostic criteria used across groups. In the development of this instrument there will be input from psychologists, psycholinguists, social workers, movement therapists, psychiatrists, program administrators, teachers, parents and other caretakers.